Boys and Sharks.. and Blood?

Boys are like sharks. They don't want a girl until someone has taken a bite out of her. Then they smell the blood and they all come. Also interesting... it seems that when a girls on her period the girl seems to attract them more then. Is it because she's crazier then, and there's something attractive about a woman who has no bearings? Or there's some sort of pheromone waft? The boy shark thing goes a step deeper here.